1. Field of Invention
The present invention has to do with improvement of variable capacitors commonly referred to as "piston trimmers" and to a preferred method of achieving the structure of the improved capacitor. More specifically, the present invention relates to a new type of resilient, deformable, resinous, tubular liner between a rigid dielectric tube supporting axially spaced capacitor electrodes and a cylindrical threaded piston. The invention also relates to a preferred method of securely engaging the liner in position.
2. Description of the Prior Art
Variable capacitors of the piston trimmer type are well known in the prior art. Such devices have been characteristically composed of a molded tubular dielectric tubular body, commonly composed of polystyrene having a pair of terminals connected to axially spaced capacitor electrodes on the outside wall of the body. The body has been threaded internally and a threaded screw having matching threads moves a metallic piston within the tubular body by virtue of threaded engagement with the tubular body. In a typical construction, the conductive piston positioned within the tubular dielectric body is electrically connected to one of the pair of terminals, electrically connecting it to one of the capacitor plates or electrodes. The piston is axially moved within the tubular dielectric body like a screw using a screw driver slot in its end, and, as it moves, it varies the capacitance of the trimmer capacitor.
In the prior art, as recited in U.S. Pat. No. 3,938,012, a major problem with this type of piston trimmer has been unreliable and inaccurate capacitance value of the trimmer. The presence of air voids between the crest of the threads of the piston which had been a problem in prior designs was solved in part according to this patent by the use of a liner member provided by inserting a very thin sheet of deformable dielectric material, usually with a metallized surface for engagement by the threaded piston, between the piston and the tubular body. Because of the formation of the liner member from an essentially flat dielectric sheet rolled to conform to the space, an axial split between the edges of the dielectric material is frequently produced, thus introducing an air space or an air gap between the piston and the electrode of the opposite electrical potential. Also, the dielectric is usually quite thin, on the order of 0.005 inches, and damage to the dielectric due to its flat shape and misalignment of some sort within the rigid dielectric member is a possibility. Structural damage can have severe effects upon performance or render the capacitor inoperative, or at least inaccurate. Also, cavities between the piston threads and the dielectric material have remained a problem. Entrapment of air or foreign substances within these spaces or in the gap between the ends of the sheet may cause temporary or permanent alteration of the parameters of the device.